extdiff: prevent exception on double-translation
The docstring is translated twice: once when used as a format string,
and once on display. The second translation fails when the first
translation introduces non-ASCII characters in the string.
The problem is that the gettext module calls unicode(message) on the
string, i.e., it decodes it to a Unicode string using the ASCII
encoding (the default encoding). By translating it into a Unicode
string here, the unicode() call becomes a noop.